FIELD OF THE INVENTION
The invention relates to a method and a comparison circuit for the detection of an input signal level exceeding a threshold value with a current-saving circuit.
BACKGROUND OF THE INVENTION
The limited capacity of power sources, ie. batteries, and their relatively short operating time between charging periods is a known problem with mobile telephones. In an attempt to make the operating time, ie. the time between chargings, of a mobile telephone longer, all circuits of the telephone are switched off when they are not used. However, some parts of the telephone's power supply circuitry are continuously connected to the battery and, therefore, in present devices, there is a continuous discharge current of about 400 to 500 uA from the battery. In addition, there is the self-discharge current of about 100 uA discharging from the battery. When a battery pack with the capacity of, say, 400 mAh is attached to the telephone, it will discharge in about 28 days even if the telephone is not used (400 mAh/(500 g A+100 uA)=667 h=28 days).
One of the parts in the power supply circuitry is a voltage comparator which has to be capable of accurate detection of the input voltage, ie. voltage comparison. Then the voltage of the analog input pin has to be detected very accurately to start a certain integrated circuit and then also the threshold value of the input signal detector has to be very accurate. The device should not function without this certain voltage in the pin and, on the other hand, the device should be turned on immediately after this certain voltage becomes available.
Appropriate prior art comparison circuits will be studied with the help of an example, with reference to the enclosed FIGS. 1 and 2, of which FIG. 1 shows an accurate detection method, or a comparison circuit, for an input signal and Figure its signals as a function of the input signal voltage level. FIG. 1 shows a comparator 1 which compares the input signal VTN to an accurate reference voltage VREF. The comparator 1 switches the output voltage VOUT on when the input voltage VIN exceeds the reference voltage VREF. FIG. 2 shows signals of the accurate detection of input signal as a function of the input signal voltage level. In the drawing, the input voltage VIN is represented by curve 2 and the output voltage VOUT by curve 3. The comparator 1 switches the output voltage VOUT on when the input voltage VIN exceeds the reference voltage VREF.
When a mobile telephone uses for voltage detection a method like the one described above, the discharge current of the battery is far too big for a lithium battery. It would be interesting to use a lithium battery but, depending on the case, a lithium battery can be continuously discharged with a current of 10 to 100 uA, at the most. In prior art solutions, the use of lithium batteries is not possible without a hardware switch or a similar circuitry.
SUMMARY OF THE INVENTION
It is an object of the invention to decrease the discharge current explained above and, to that end, we will now further study the comparator circuit mentioned above.
In accordance with the objective, it is characteristic of the inventive method that the input signal is first taken to a first
comparing element that generates an intermediate output signal in its output only when the input signal level exceeds the coarse reference voltage; the intermediate output signal switches on a second comparing element; and the input
5 signal is also taken to the second comparing element which generates a detection signal in its output only when the input signal level exceeds the accurate reference voltage. Then it is assumed that the absolute value of the accurate reference voltage is bigger than the absolute value of the coarse
10 reference voltage. Advantageously, the intermediate output signal also switches on the accurate reference voltage generator.
A comparison circuit that implements the method according to the invention is advantageously so implemented that
15 the first comparing element is a transistor switch, in which case the coarse reference voltage is the threshold voltage of the transistor. The transistor switch may be an NPN transistor with a threshold voltage of about 0.7 V; then the electric circuit of the transistor is so dimensioned that the
20 current it takes from the power source is small, advantageously about 100 uA, when the input voltage is smaller than the coarse reference voltage.
It is an advantage of the inventive solution that a lithium battery can be used as a power source for the device.
BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention are illustrated by the following implementation example which is explained in detail with reference to the enclosed 30 drawings, where:
FIGS. 1 and 2 illustrate prior an methods described above,
FIG. 3 shows a block diagram of the accurate detector solution for the input signal according to the invention, 35 FIG. 4 shows signals of the detector shown in FIG. 3 as a function of the input signal voltage level, and
FIG. 5 shows the circuit diagram of the detector shown in FIG. 3.
DETAILED DESCRIPTION OF THE 40 INVENTION
In the inventive method, an input signal exceeding the input signal level threshold value is detected by means of a comparing element and a reference voltage brought to it 45 The method can be implemented in phases; hence, referring to FIGS. 3 and 4:
a) an input signal VIN (5) is brought to a first comparing element, or, a transistor 4 (POWER DOWN) which generates an intermediate output signal PWD (6) in its
50 output only when the level of the input signal VIN (5) exceeds the coarse reference voltage VREFl;
b) the intermediate output signal PWD (6) switches a second comparing element 1 (COMPARATOR) and the accurate reference voltage VREF2 generator (9, in FIG.
55 5) on; and
c) the input signal VTN (5) is brought to the second comparing element 1 which generates in its output a detection signal VOUT (7) only when the level of the input signal VIN (5) exceeds the accurate reference
60 voltage VREF2.
The absolute value of the accurate reference voltage VREF2 is higher than the absolute value of the coarse reference voltage VREFl, which means, as one skilled in the art will understand, that the voltage logic can be positive or
65 negative. In the following examples a positive voltage logic will be assumed, ie. the voltages are positive in comparison with the ground, or zero, of the comparison circuit.